1. Field of the Invention
This invention relates to an electrolytic process for the treatment of strip metal, and more particularly to an improved cathode structure for use in an electrolytic process in which the positively-charged anode and negatively-charged cathode are separated by an ion-permeable membrane.
2. Description of the Prior Art
Electrochemical or electrolytic processes for the continuous treatment of a running length of strip metal, and apparatus for the performance of such processes, are known in which anode and cathode means are immersed in anolyte and catholyte solutions, respectively, with the solutions being confined to contiguous chambers separated by an ion-permeable wall or membrane. A method of producing galvanized sheet metal having a zinc coating on one side only is disclosed in U.S. Pat. No. 3,988,,216, assigned to the assignee of the present invention. According to this prior art patent, a strip of metal which has been previously coated on both sides is drawn through a first electrolyte solution between an anode immersed in the bath and a cathode immersed in a second electrolyte solution which is kept separated from the first solution by a perm-selective anion membrane. By applying negative current to the cathode and positive current to the anode, zinc is removed from the side of the strip facing the cathode and a substantially equal amount of zinc is simultaneously plated onto the side facing the anode.
Electrolytic treatment apparatus is also known in which anolyte and catholyte solutions are continuously flowed through adjacent chambers separated by an ion-permeable membrane during operation, one such apparatus being shown, for example, in U.S. Pat. No. 3,945,892.
In the production of galvanized strip steel, relatively high current densities are required in order to plate the strip at a commercially acceptable rate. Such strip may be up to six feet, or more, in width. This width, combined with the high speed of the strip through the apparatus, requires the use of relatively large anode and cathode surface areas and high current densities in order to effectively plate the strip. The high current densities and large electrode areas result in the generation of substantial amounts of heat which tends to heat the electrolyte solutions in which the anode and cathode are immersed.
Ion-permeable membranes for use in electrolytic processes are commercially available and conventionally are formed materials such as thermoplastic synthetic resin materials which are heat-sensitive and very delicate when formed into a thin sheet or membrane. The heat which can build up in the electrolyte solutions during the high speed electrolytic treatment of strip metal has in the past caused serious problems in the use of the heat sensitive ion-permeable membranes in such apparatus.
In the one-side galvanized process of U.S. Pat. No. 3,988,216, the anode is immersed in an acidic electrolyte solution, or anolyte, and the cathode in a basic electrolyte solution, or catholyte. When the strip is passed between the anode and cathode, zinc coating on the side of the strip adjacent the cathode is oxidized to zinc ions which go into solution, while a substantially equivalent amount of zinc ions are reduced to zinc metal and deposited from the solution on the side of the strip facing the anode. Water disassociates at the anode and the cathode, with hydroxyl ions and hydrogen gas being generated at the cathode and hydrogen ions and oxygen gas being generated at the anode. The hydroxyl ions carry the electrical current through the ion-permeable membrane where they reunite with the hydrogen ions to re-form water. However, the hydrogen gas generated at the surface of the cathode tends to interfere with the electrolytic action of the apparatus, particularly when the gas is permitted to accumulate and form bubbles on the surface of the cathode.
It is, therefore, the primary object of the present invention to provide an improved electrolytic process for use in the continuous treatment of strip metal, and to provide an improved cathode structure for use in such electrolytic process.
It is a further object of the present invention to provide an improved cathode structure for use in an electrolytic process in which the anode and cathode are immersed in separate electrolytic solutions separated by an ion-permeable membrane.
Another object of the invention is to provide such an improved cathode structure for use in the production of one-side galvanized sheet or strip material and having improved means for cooling the cathode and removing hydrogen gas from the surface of the cathode.
Another object of the invention is to provide an improved cathode structure having means for circulating the anolyte solution over the surface of the cathode at a rate sufficient to effectively flush hydrogen gas from the cathode surface and to cool the surface and the adjacent ion-permeable membrane.